Sheathed Element Glow Plug Having a Combustion Chamber Pressure Sensor

ABSTRACT

A sheathed element glow plug includes an integrated combustion chamber pressure sensor for a self-igniting internal combustion engine for measuring combustion chamber pressures, in particular for combustion chamber signal-based engine control. The sheathed element glow plug has a heating element and a glow plug housing, as well as a glow plug axis. The glow plug housing has a receptacle area for receiving the heating element, a housing body, and at least one flexibility area situated between the housing body and the receptacle area. At least one force measuring element is installed in the glow plug housing.

FIELD OF THE INVENTION

The present invention relates to a sheathed element glow plug having an integrated combustion chamber pressure sensor. Sheathed element glow plugs of this type are used in particular in self-igniting internal combustion engines for measuring a combustion chamber pressure.

BACKGROUND INFORMATION

As a result of increasingly strict exhaust gas legislation, in particular for diesel engines, the demands for reduced emissions of harmful substances by self-igniting internal combustion engines are becoming stricter. Today's engine management systems are expected to ensure low emissions of harmful substances and low fuel consumption, in addition to having a long service life at the same time. Combustion may be optimized in the combustion chamber of a diesel engine in particular by using regulated injection of fuel. Regulated injection may be controlled in particular by electronic engine control units, which are already routinely used in today's motor vehicles. Successful operation of a combustion signal-based control system (CSC) depends, however, on the availability of pressure sensors that are industrially manufacturable and meet high demands regarding price, reliability, accuracy, and compactness. At this time, measuring devices having so-called “stand-alone sensors” are widely used. However, for using such devices, a separate bore hole must be provided in the cylinder head wall, which is not always feasible for reasons of space and it also means additional assembly effort, including additional operations. In particular, in today's four-valve internal combustion engines, making additional bore holes is hardly implementable in practice due to the extremely tight space constraints. Furthermore, in general, systems of this type are relatively expensive and the service life of systems of this type is significantly shorter than that of a typical vehicle, mostly due to the high operating temperatures.

Therefore, there have been conventional approaches to integrate combustion chamber pressure sensors into existing components of the cylinder head. For example, there are approaches to integrate combustion chamber pressure sensors in sheathed element glow plugs. For example, German Published Patent Application No. 196 80 912 describes a device and a method for detecting the cylinder pressure in a diesel engine. The device has a pressure sensor, a heating section of a glow plug, which is installed in the interior of a cylinder of the diesel engine and can be acted upon by the cylinder pressure, and a fastening element for fastening the heating section in a body of the glow plug. The pressure sensor is situated between the heating section and the fastening element of the glow plug. The cylinder pressure is transmitted to the pressure sensor via the heating section.

The device described in German Published Patent Application No. 196 80 912 has, however, numerous practical disadvantages. In particular, in the device, the housing of the sheathed element glow plug is essentially fixedly connected to the cylinder head, while the heating section introduces the force and, in doing so, performs a relative movement, however comparatively small it may be, with respect to the housing. This in turn may result in friction and thus in a corruption of the combustion chamber pressure signal; in particular there is the additional risk of soot deposition on the guide between the stationary part (i.e., the part fixedly connected to the cylinder head) and the moving part of the sheathed element glow plug. Another disadvantage of the device described in German Published Patent Application No. 196 80 912 is that the housing and the force-transmitting components expand differently. These different expansions, which are caused by the temperature differences during the use of the internal combustion engine and different thermal expansion properties of the components involved, cause strong fluctuations in the pre-tension of the sensor used, which in turn may result in service life problems. Furthermore, the device described in German Published Patent Application No. 196 80 912 has the disadvantage that the pressure sensor is directly connected to the heating section and is thus exposed to high thermal stresses and temperature fluctuations. In addition, assembly of the above-described device is relatively complex.

SUMMARY

Therefore, according to example embodiments of the present invention, a sheathed element glow plug for a self-igniting internal combustion engine has an integrated combustion chamber pressure sensor and which avoids the disadvantages of the devices as described above. In particular, the sheathed element glow plug permits the fluctuations in the sensor pre-tension, caused by temperature, and the mechanical friction of individual components of the sheathed element glow plug during operation, which, as described above, may result in problems in certain conventional devices to be minimized. The sheathed element glow plug for a self-igniting internal combustion engine has a heating element, a glow plug housing, and a glow plug axis. Example embodiments of the present invention provide a fixed and, e.g., fully gas-tight connection established between the glow plug housing and the heating element. The functionality of the combustion chamber pressure sensor is achieved through the flexibility of the glow plug housing in the area between its combustion chamber-side end and its end screwed into the cylinder head.

The sheathed element glow plug has a receptacle area for receiving the heating element as well as a housing body and at least one flexibility area situated between the housing body and the receptacle area. The flexibility area may be configured such that it has at least one area in which the glow plug housing has a lower rigidity parallel to the glow plug axis than in the area of the housing body. At least one force measuring element, e.g., a force measuring element which may generate an electrical signal as a function of a force exerted on the at least one force measuring element, is provided in the glow plug housing. Basically a conventional force measuring elements, based on any principle, for example, piezoelectric force measuring elements of almost any design or also capacitive force measuring elements or force measuring with the aid of strain gages, may be used. The at least one force measuring element may be installed in the housing body.

The pressure prevailing in the combustion chamber must be transmitted to the at least one force measuring element as a force. This force may be transmitted, for example, directly from the heating element to the at least one force measuring element or indirectly, for example, via the glow plug housing or sections of the glow plug housing. Alternatively or additionally, for the purpose of pressure transmission the sheathed element glow plug may also have at least one separate force transmission element for transmitting the combustion chamber pressure to the at least one force measuring element, e.g., for transmitting a force from the heating element to the at least one force measuring element. For example, this at least one separate force transmission element may be a pressure rod, e.g., an essentially cylindrical pressure rod, and/or a pressure sleeve, e.g., an essentially cylindrical sleeve-shaped pressure sleeve. By “essentially” it is to be understood here that a slight deviation from a cylindrical shape or cylindrical sleeve shape is also possible, for example, a slightly conical shape, which is adapted, for example, to the design of the sheathed element glow plug or of an interior of the sheathed element glow plug. The heating element projects into the combustion chamber of the internal combustion engine and applies a pressure corresponding to the combustion chamber pressure to a pressure surface, for example, an end face. This pressure is converted by the heating element into a force, which is transferred from the heating element to the sheathed element glow plug. The at least one force transmission element transmits this force directly or indirectly (i.e., with or without additional intermediary elements) from the heating element to the at least one force measuring element, where this force is converted into an electrical signal, which may be read by an appropriate electronic circuit and made available, for example, to an engine controller. In this manner, up-to-the-minute information about the combustion chamber pressure may be generated.

The at least one flexibility area may be configured in different manners. Its function is to be able to displace the entire front part of the sheathed element glow plug, i.e., the part facing the combustion chamber of the internal combustion engine which includes the receptacle area and the heating element, along the glow plug axis when a pressure is applied to the heating element due to the combustion chamber pressure, and thus to apply a corresponding force and thus a pre-tension to the at least one force transmission element. In contrast, the housing body expands very little or not at all and remains substantially rigid during this elastic deflection. The pressure force, which is introduced into the sheathed element glow plug via the at least one force transmission element, may thus be detected by the at least one force measuring element. This pressure force differs from the total force introduced into the sheathed element glow plug via the combustion chamber pressure only by a substantially constant factor, which is a function of the rigidity of the glow plug housing in the receptacle area and in the flexibility area of the glow plug housing. The rigidity of the at least one force transmission element also affects this essentially constant factor.

The flexibility area may have an undulation or a bellows having at least one fold turned to the inside of the glow plug housing or to the outside. Alternatively or additionally, the at least one flexibility area may also have at least one area having a small wall thickness of the glow plug housing, in particular a wall thickness that is less than in the adjacent areas of the glow plug housing or than in the entire rest of the glow plug housing. This arrangement also results in a reduced rigidity of the glow plug housing parallel to the glow plug axis in the flexibility area. Alternatively or additionally, an elastic element, for example, a spring element, e.g., a helical spring or a similar spring element or also an elastic element made of metallic material or a plastic, for example, an elastomer, may also be used, which ensures flexibility in the at least one flexibility area parallel to the glow plug axis. In general, an element, e.g., a material, having a low modulus of elasticity may also be used. A low modulus of elasticity is to be understood here in particular as a modulus of elasticity which is smaller than the moduli of elasticity of the surrounding wall areas or of the entire glow plug housing.

The heating element may be fixedly and pressure-tightly connected to the glow plug housing in the receptacle area, e.g., via press-fitting. The glow plug housing is connected to the cylinder head, e.g., via a threaded connection. For this purpose, the sheathed element glow plug may also have at least one external thread for connecting the sheathed element glow plug to the cylinder head of the internal combustion engine.

This at least one external thread may be a component of the housing body of the sheathed element glow plug. Between the receptacle area for receiving the heating element and the connection to the cylinder head, there is at least one flexibility area, for example, in the form of a metallic bellows, in which the glow plug housing has minimum rigidity parallel to the glow plug axis.

The at least one force transmission element is supported, e.g., as far to the front toward the combustion chamber as possible by the glow plug housing or even directly by the heating element. On its opposite end, the at least one force transmission element is supported, directly or indirectly, by the at least one force measuring element, so that a force is transmissible, as described above, from the at least one heating element to the at least one force measuring element. When installed, the glow plug housing may be tensile stressed and the at least one force transmission element may be compression stressed. This stressing (pre-tension) may take place, for example, with the aid of a thread or a caulking of the at least one force measuring element in the glow plug housing, e.g., in the housing body. An advantage of the flexibility area is that different thermal expansions of the glow plug housing and of the at least one force transmission element are compensated by the flexibility of the housing in the area of the at least one flexibility area and thus cause only a relatively small fluctuation of the pre-tension force exerted on the at least one force measuring element. This results in better signal quality and avoids signal correction in the different operating states of the internal combustion engine which usually also result in corresponding temperature fluctuations. Fluctuations in the outside temperatures are also at least partially compensated. In rest operation of the internal combustion engine, e.g., no axial forces act on the heating element, the connection between heating element and glow plug housing being unstressed in. rest operation.

As described above, one end of the at least one force transmission element may be supported directly or indirectly (for example, via an intermediary element) by the at least one force measuring element. The other end of the transmission element may be supported directly by the heating element, for example, or, alternatively or additionally, by the at least one flexibility area. For example, the at least one force transmission element may be supported by an undulation of the flexibility area directed toward the inside of the glow plug housing. Alternatively or additionally, the at least one force measuring element may also be supported by an area of the glow plug housing which is situated between the at least one flexibility area and the at least one heating element. For example, for this purpose, at least one additional supporting element may be used, which is used for supporting the at least one force transmission element on the glow plug housing in the area between the flexibility area and the heating element. For example, a circular disk may be used, whose periphery is connected, for example, screwed or caulked, to the wall of the glow plug housing. These options for supporting the at least one force transmission element cause the force transmission element to be supported, as described above, as far to the front toward the combustion chamber as possible, causing minimum tensions to occur in the rigid area of the glow plug housing which faces away from the combustion chamber. The at least one flexibility area may be directly adjacent to the heating element in the glow plug housing or, alternatively or additionally, a gap between the heating element and the flexibility area is additionally filled with a filling material. The effect of this refinement is that no excessive flexibility occurs prior to the introduction of force from the heating element to the at least one force transmission element. For this purpose, the filling material may be, for example, a highly rigid and, e.g., poorly heat-conducting material. This refinement causes the most direct force transmission possible from the heating element to the at least one force transmission element, which further improves the force transmission function of the combustion chamber pressure onto the at least one force measuring element.

The power may be supplied to the heating element for example via one of the steel terminal studs located centrally near the glow plug axis. The use of flexible glow wire power supply leads is, however, also possible.

The sheathed element glow plug having an integrated combustion chamber pressure sensor has numerous advantages compared to conventional devices. One important advantage is the independence of the combustion chamber pressure signal from the operating temperature of the internal combustion engine because temperature fluctuations and related differing material expansions are compensated in an optimum manner. Another advantage is that an almost constant force transmission function is ensured. This means, e.g., that the combustion chamber pressure is transmitted to the at least one force measuring element in an identical or similar manner in almost all ranges of the combustion chamber pressure and thus in almost all operating ranges of the internal combustion engine. The force transmission factor by which the electrical signal of the at least one force measuring element is to be multiplied to deduce the actual combustion chamber pressure from this signal is thus largely independent of the operating state of the internal combustion engine. Additional corrections which involve complex calculations and must include, for example, appropriate correction functions, etc., may thus be avoided. The electrical signal of the at least one force measuring element may thus be used directly or after only minor electronic processing for a corresponding engine control, for example, for engine control based on the combustion chamber pressure signal.

Example embodiments of the present invention are described in greater detail with reference to the Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sheathed element glow plug according to an example embodiment of the present invention having an integrated combustion chamber pressure sensor;

FIG. 2 shows a simulated curve of a sensor signal of a force measuring element, compared to the combustion chamber pressure, at different crankshaft positions;

FIG. 3 shows a combustion chamber pressure sensor according to an example embodiment of the present invention, and

FIG. 4 shows a combustion chamber pressure sensor according to an example embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 shows a sheathed element glow plug 110 according to an example embodiment of the present invention having an integrated combustion chamber pressure sensor. Sheathed element glow plug 110 has a heating element 112 and a glow plug housing 114. Glow plug housing 114 is subdivided into three areas: a receptacle area 116 facing the combustion chamber for receiving heating element 114, a flexibility area 118, and a housing body 120 situated on the side of sheathed element glow plug 110 facing away from the combustion chamber.

Heating element 112 is designed as a ceramic heating element 112 in this exemplary embodiment. However, other designs of heating elements 112 are also possible. In this exemplary embodiment, electrical power is applied to heating element 112 via a steel terminal stud 122. In this exemplary embodiment, according to FIG. 1, steel terminal stud 122 is designed as a solid central stud, which extends axially along a glow plug axis 124 through glow plug housing 114. Sheathed element glow plug 110 has a threaded connection 126 on its end facing away from the combustion chamber. Steel terminal stud 122 axially projects out from glow plug housing 114 through this threaded connection 126 and is connected to an appropriate electrical power supply. Furthermore, glow plug housing 114 has an external thread 128 in housing body 120. Using this external thread 128, sheathed element glow plug 110 may be screwed into a cylinder head such that heating element 112 projects into the combustion chamber of the internal combustion engine. In flexibility area 118, glow plug housing 114 is provided with a fold 130 in which glow plug housing 114 has an inward contraction. Fold 130 thus acts as a metallic bellows having a single fold and provides glow plug housing 114 with a lower rigidity parallel to glow plug axis 124 in flexibility area 118 than in housing body 120.

Heating element 112 is connected to glow plug housing 114 by press-fitting 132 in receptacle area 116 in this exemplary embodiment. A gas-tight connection is thus created in receptacle area 116, which ensures that combustion chamber gases cannot penetrate into the interior of sheathed element glow plug 110. In receptacle area 116, heating element 112 is pressed into glow plug housing 114. This creates a gap 134 between heating element 112 and fold 130. It may be provided to keep this gap 134 as small as possible, to possibly make it disappear or to fill this gap 134 with a highly rigid and/or poorly heat-conducting filling material. In this manner, the force transfer, which is described below, is further improved, while heat transfer from heating element 112 to other components in the interior of sheathed element glow plug 110 is prevented.

Furthermore, a force measuring element 136, which has an annular shape in this exemplary embodiment, in the form of an annular piezoelectric element, is installed in housing body 120 of glow plug housing 114. The electrical leads of this force measuring element 136 are not illustrated in FIG. 1, and may exit glow plug housing 114 axially, for example, parallel to steel terminal stud 122 through threaded connection 126 and be connected to an appropriate electrical electronic analyzer circuit. In this exemplary embodiment, force measuring element 136 is surrounded by two spacing sleeves 138, for example, spacing sleeves of a cylindrical sleeve shape, in particular spacing sleeves made of a highly rigid material, for example, steel. During assembly, spacing sleeve 138 on the combustion chamber side is first inserted into glow plug housing 114 from the end of sheathed element glow plug 110 facing away from the combustion chamber, then force measuring element 136, and subsequently second spacing sleeve 138, are inserted. Subsequently glow plug housing 114 is screwed in using threaded connection 126. A pre-tension is thus applied to force measuring element 136.

Furthermore, a force transmission element 140 is introduced into glow plug housing 114. In this exemplary embodiment, force transmission element 140 has a sleeve-shaped design and also includes, like force measuring element 136, steel terminal stud 122 on the periphery. In this exemplary embodiment, force transmission element 140 has a slight conicity and a slightly smaller external diameter on the combustion chamber side than on the side facing away from the combustion chamber. On the combustion chamber side force transmission element 140 is supported by fold 130 and on the side facing away from the combustion chamber by spacing sleeve 138. Therefore, in this exemplary embodiment, force transmission element 140 is indirectly supported by force measuring element 136.

On its side facing the combustion chamber, heating element 112 has a hydraulic pressure surface 142. On this pressure surface 142, the combustion chamber pressure is converted into a force F (labeled with reference numeral 144 in FIG. 1) exerted on heating element 112. Force transmission element 140 transmits this force 144 onto force measuring element 136, where it is converted into an electrical signal. From this electrical signal, a conclusion can be drawn about the combustion chamber pressure. The transmission of force 144 from heating element 112 to force measuring element 136, however, is not complete, but must be multiplied by a factor which is less than 1. In the ideal case, this transmission factor attains the exact value of 1. The fact that the transmission is incomplete is explained by the fact that forces are absorbed by glow plug housing 114. The advantage of the design of sheathed element glow plug 110 illustrated in FIG. 1 is, however, that force 144 in this case results in a negligible deformation of glow plug housing 114 outside flexibility area 118. Substantially, only receptacle area 116 is displaced by force 144 axially with respect to housing body 120, glow plug housing 114 being elastically deflected axially in the region of flexibility area 118. This ensures that force 144 is almost completely transmitted from heating element 112 to force measuring element 136. Furthermore, fold 130 also absorbs thermal stresses, so that substantially a constant pre-tension is applied to force measuring element 136 even at different operating temperatures.

The transmission of force 144 onto force measuring element 136 in the system according to FIG. 1 is schematically illustrated in FIG. 2 in the form of simulation data, where the x axis, here labeled with φ, denotes the position of the crankshaft in degrees, the left-hand y axis denotes combustion chamber pressure p in arbitrary units, and the right-hand y axis denotes force F displayed by force measuring element 136 in arbitrary units. For a simulation, an operating point at 2000 rpm and an effective mean pressure (PME) of one bar are assumed. Upper curve 210, which refers to the left-hand y axis, shows the variation of the combustion chamber pressure. Lower curve 212, which refers to the right-hand y axis, shows the electrical signal of force measuring element 136. As is apparent from FIG. 2, sensor signal 212 is to be multiplied by an appropriate factor for a conclusion to be drawn about combustion chamber pressure 210 from this sensor signal 212. This factor substantially includes the material characteristics and the design of sheathed element glow plug 110.

FIG. 3 shows a sheathed element glow plug 110 according to an example embodiment of the present invention. Again, sheathed element glow plug 110 has a glow plug housing 114, which is subdivided into a receptacle area 116, a flexibility area 118, and a housing body 120. In receptacle area 116, a heating element 112 is again pressed into glow plug housing 114 by press-fitting 132. As also in the exemplary embodiment according to FIG. 1, sheathed element glow plug 110 in the exemplary embodiment of FIG. 3 also has a fold 130 in flexibility area 118. The design of this fold 130 is basically comparable to the design of fold 130 in the exemplary embodiment of FIG. 1. A gap 134 is again formed between fold 130 and heating element 112. As in the exemplary embodiment of FIG. 1, glow plug housing 114 again has an external thread 128 for fastening sheathed element glow plug 110 in a cylinder head.

The difference between the exemplary embodiment of FIG. 3 and the exemplary embodiment of FIG. 1 is substantially in the design of force measuring element 136 and the design and positioning of force transmission element 140. In the exemplary embodiment of FIG. 3, force transmission element 140 is designed in the form of a cylindrical disk, whose end facing away from the combustion chamber is inserted into housing body 120. This force measuring element 136 is also secured and pre-tensioned by a threaded connection 126. No spacing sleeves 138 are used in this exemplary embodiment according to FIG. 3.

Furthermore, in the exemplary embodiment according to FIG. 3, force transmission element 140 has a rod-shaped, rather than sleeve-shaped, design. Force transmission element 140 is inserted into glow plug housing 114 along glow plug axis 124. On its end facing away from the combustion chamber, force transmission element 140 is supported centrally by the combustion chamber-side end face of force measuring element 136. On its end facing the combustion chamber, rod-shaped force transmission element 140 is supported by the wall of gap 134. For this purpose, an additional, circular disk-shaped support element 310 is inserted into receptacle area 116. This support element 310 may be caulked or screwed to the wall of glow plug housing 114 in receptacle area 116, for example. Other types of attachment are also conceivable. Support element 310 causes a force to be transmitted from heating element 112 to force measuring element 136 via the wall of glow plug housing 114 in receptacle area 116, via support element 310 and finally via rod-shaped force transmission element 140. AN advantage of an indirect force transmission from heating element 112 to force transmission element 140 via support element 310 is substantially that no heat is transferred directly from heating element 112 to force transmission element 140. Such heat transfer by force transmission element 140 (which may be made of metal, for example) also to force measuring element 136, might result, for example, in temperature fluctuations in force measuring element 136, which would negatively affect the signal quality.

In the exemplary embodiment of FIG. 3, the power lead to heating element 112 is not illustrated. Since in this exemplary embodiment the area along glow plug axis 124 is essentially filled by rod-shaped force transmission element 140, there is not space here for a steel terminal stud 122 according to the exemplary embodiment of FIG. 1. Instead, in the exemplary embodiment of FIG. 3, a glow wire power supply lead is used, which passes by elements 310 and 136 via appropriate bore holes in the support element or appropriate bore holes or grooves in the wall of glow plug housing 114 and exits to the outside via threaded connection 126.

FIG. 4 shows a sheathed element glow plug 110 without a separate force transmission element 140. Force 144 is directly transferred here from heating element 112 to force measuring element 136. This transfer preferably takes place with the aid of a cylindrical extension 410 of heating element 112 situated on the side of heating element 112 facing away from the combustion chamber. Otherwise the functionality and design of sheathed element glow plug 110 is similar to the exemplary embodiment according to FIG. 3. 

1-11. (canceled)
 12. A sheathed element glow plug for a self-igniting internal combustion engine, comprising: a heating device; a glow plug housing; and a glow plug axis; wherein the glow plug housing includes a receptacle area configured to receive the heating device, a housing body and at least one flexibility area arranged between the housing body and the receptacle area, at least one force measuring device arranged in the glow plug housing.
 13. The sheathed element glow plug according to claim 12, further comprising at least one force transmission device configured to transmit a combustion chamber pressure to the at least one force measuring device.
 14. The sheathed element glow plug according to claim 12, wherein the at least one flexibility area includes at least one area in which the glow plug housing has a lower rigidity parallel to the glow plug axis than in an area of the housing body.
 15. The sheathed element glow plug according to claim 12, wherein the at least one flexibility area includes at least one of: (a) an undulation; (b) a bellows having at least one fold turned to one of (i) an inside and (ii) an outside of the glow plug housing; (c) an area having a small wall thickness of the glow plug housing; (d) an elastic device; and (e) an device having a low modulus of elasticity.
 16. The sheathed element glow plug according to claim 12, wherein the at least one force measuring device is accommodated in the housing body.
 17. The sheathed element glow plug according to claim 12, wherein the at least one force transmission device includes at least one of: (a) a pressure rod; (b) a substantially cylindrical pressure rod; (c) a pressure sleeve; and (d) a substantially cylindrical sleeve-shaped pressure sleeve.
 18. The sheathed element glow plug according to claim 13, wherein one end of the at least one force transmission device is supported by the at least one force measuring device and the other end is supported by at least one of: (a) the heating device; (b) the at least one flexibility area; and (c) an area of the glow plug housing arranged between the at least one flexibility area and the at least one heating device.
 19. The sheathed element glow plug according to claim 13, wherein one end of the at least one force transmission device is supported by the at least one force measuring device and the other end is supported by an area of the glow plug housing arranged between the at least one flexibility area and the at least one heating device, the sheathed element glow plug further comprising at least one supporting device configured to support the at least one force transmission device on the glow plug housing.
 20. The sheathed element glow plug according to claim 12, further comprising at least one external thread configured to connect the sheathed element glow plug to a cylinder head of the internal combustion engine, the at least one external thread being a component of the housing body.
 21. The sheathed element glow plug according to claim 12, wherein the at least one force measuring device is one of (a) annular- and (b) disk-shaped.
 22. The sheathed element glow plug according to claim 13, wherein the at least one force measuring device is arranged in the glow plug housing such that a tensile pre-stress is applied to at least part of the glow plug housing, and a compression pre-stress is applied to the at least one force transmission device.
 23. The sheathed element glow plug according to claim 22, wherein the at least one force measuring device is one of (a) screwed in and (b) caulked to the glow plug housing. 